The treatment of pigmentary titanium dioxide ((TiO.sub.2) in the rutile or anatase form with hydrous oxides, especially silica and alumina, is known in the art to provide resistance to chalking and discoloration in paints, good opacity in paper and good hiding power in flat paints. Typical application techniques for applying silica are taught in U.S. Pat. Nos. 2,885,366; 2,387,534; and 2,296,636. Generally, the silica is applied to the pigment by precipitation from sodium silicate with acid in an aqueous slurry of the pigment. The resulting pigments, having a dense, i.e., non-porous silica coating, have good durability but do not provide as high a degree of hiding power as is desirable in many paint compositions. Applying a loose, i.e., porous, alumina to dense silica-coated TiO.sub.2 according to the process taught in U.S. Pat. No. 3,437,502 improves the dispersibility of dense silica-coated TiO.sub.2 without sacrificing durability, but does not significantly enhance the hiding power of the dense silica-coated TiO.sub.2.
Applying a porous coating consisting essentially of alumina and silica to pigmentary titanium dioxide according to the process taught in U.S. Pat. No. 3,591,398 provides a coated pigment having generally better hiding power than the densely coated pigment, but lower durability than is desired for many applications. A further increase in hiding power is achieved by treating TiO.sub.2 with four applications consisting of porous silica and alumina followed by additional applications of porous silica and alumina, as described in U.S. Pat. No. 3,897,261. While the TiO.sub.2 pigments resulting from the multiple treatment described in U.S. Pat. No. 3,897,261 exhibit improved hiding power and somewhat improved durability, the durability achieved falls far short of that exhibited by densely coated pigments.
One method to achieve a high degree of durability and hiding power is disclosed in U.S. Pat. No. 3,649,322 wherein pigmentary TiO.sub.2 is coated with a dense skin of hydrated aluminum silicate and optionally an additional coating of porous alumina on the dense skin.
This invention provides for a pigmentary TiO.sub.2 coated in such a way as to combine the durability of dense silica coatings with the hiding power of loose hydrous coatings.
For purposes of this application pigmentary means that the material is pulverulent and of good whiteness with an average particle diameter of from about 0.15 to about 0.3 micron.